1. Field of the Invention
The present invention relates to a magnetic field generating apparatus used in a predetermined athletic time measuring system, and particularly to a magnetic field generating apparatus and magnetic field controlling method that can suitably control an electromagnetic field to be generated.
2. Description of the Related Art
At the marathon races and the like, attempts at measuring each runner's finish time have recently been made. For example, the following time measuring system is put to practical use. Specifically, a barcode is printed on a runner's number cloth and each runner's finish time is measured based on time obtained when the barcode of the runner, who has crossed the finish line, is read by a reader.
However, in such a time measuring system, since barcode reading is executed for a predetermined time after the runner crosses the finish line, a finish time, which is delayed as compared with an actual measurement, is measured. Particularly, when many runners cross the finish line at the same time, waiting time for reading the barcode is caused. This causes a problem in which their finish time, which is considerably delayed as compared with an actual measurement, is measured.
Moreover, there grows a need to measure running time including not only the finish time but also an elapsed time for each split point. However, the conventional time measuring system cannot meet such a need.
In order to solve such a problem, various kinds of time measuring systems are developed and operational tests for practical use are tried. In the new time measuring systems, a mainstream method is that each runner's running time is measured in a non-contact manner in order to measure the finish time closer to the actual measurement and enable to measure the elapsed time for each split point.
For example, there is proposed a time measuring system in which a small-sized timer device is held by each runner and running time is automatically measured by the timer device when the runner reaches a time measuring point (each split point and a finish point). In such a time measuring system, for example, an electromagnetic field is generated at a time measuring point and the electromagnetic field is detected by use of an electromagnetic induction coil included in the timer device, thereby determining the arrival to the time measuring point.
For example, Unexamined Japanese Patent Application KOKAI Publication No. H8-221627 discloses a time measuring system explained below. Namely in the time measuring system, two loop coils are arranged in parallel to sandwich a finish line on a marathon running course. Current is supplied to each loop coil from an AC power supply with a different output frequency to generate an electromagnetic field. Then, the electromagnetic induction coil detects the electromagnetic field on the first loop coil (front side seen from the runner) to obtain the frequency in accordance with the arrival of the runner to the finish point. Sequentially, the electromagnetic induction coil captures the electromagnetic field on the second loop coil to detect a change in the frequency. Then, a position where the change in the frequency is detected corresponds to the finish line. The timer device measures the running time with timing when the change in the frequency is detected.
The aforementioned document discloses the technique in which the current is supplied to the loop coils from the AC power supply to generate the electromagnetic field. However, it is difficult to say that sufficient structural components as an apparatus that generates the electromagnetic field are explained in the description.
Generally, a resonant capacitor, an ammeter and the like are required for the magnetic field generating apparatus that generates the electromagnetic field in addition to the aforementioned AC power supply and the loop coils. Namely, in the aforementioned document, the structural components necessary for the magnetic field generating apparatus and sufficient explanation thereof are omitted.
The following will explain the general magnetic field generating apparatus with reference to FIG. 8A and FIG. 8B. FIGS. 8A and 8B are schematic views each explaining the structure of the conventional magnetic field generating apparatus.
A magnetic field generating apparatus illustrated in FIG. 8A includes an AC power supply 101 having an amplifier, a fixed resonant capacitor 102, and a loop coil 103.
A value (capacitance) of the resonant capacitor 102 is set by a skilled operator before the start of the race so that the magnetic field generating apparatus is adjusted in such a way to generate a suitable electromagnetic field on the loop coil 103. In other words, a condition of a location where the loop coil 103 is mounted, a size of the loop coil 103, the number of turns thereof, a length of a lead portion L and the like are taken into consideration to set the value of the resonant capacitor 102.
While, a magnetic field generating apparatus illustrated in FIG. 8B includes an AC power supply 101, multiple resonant capacitors 102, a loop coil 103, a selection switch 104, and an ammeter 105.
The magnetic field generating apparatus can select the resonant capacitor 102 with an arbitrary capacitance by a manual operation of the selection switch 104. Namely, the operator suitably selects any resonant capacitor 102 as checking a current value flowing into the loop coil 103 by use of the ammeter 105 so that an adjustment is made to generate an appropriate electromagnetic field on the loop coil 103.
For this reason, the electromagnetic field on the loop coil 103 can be easily adjusted to a certain degree by even an operator who is not skilled.
However, when such a magnetic field generating apparatus is actually used, load conditions (deformation of the loop coil 103, extension of the lead portion L, environmental conditions (water, humidity, etc.)) at the location where the loop coil 103 is mounted are changed halfway in many cases. Moreover, it is known that a resonance point (series resonating frequency formed by the resonant capacitor 102 and the loop coil 103) is easily varied, depending on the characteristic of the loop coil 103.
In this way, when the load conditions are changed halfway to fail in satisfying an optimal driving condition, it is substantially impossible for the magnetic field generating apparatus illustrated in FIG. 8A to readjust the electromagnetic field on the loop coil 103.
While, the magnetic field generating apparatus illustrated in FIG. 8B can readjust the electromagnetic field even when the load conditions are changed. However, the operator must always monitor the ammeter 105 and appropriately operate the selection switch 104 during the race. Accordingly, extremely complicated operations are required to maintain the electromagnetic field in a suitable state. Furthermore, when much time is taken to operate the selection switch 104, the electromagnetic field on the loop coil 103 is in an unsuitable state (state that a suitable electromagnetic field is not generated), so that the electromagnetic field cannot be detected by the timer device (electromagnetic induction coil).